It is the world's deadliest outbreak to date and has centred on Guinea, Liberia and Sierra Leone. There have also been two cases in the Nigerian city of Lagos, where eight people are currently in quarantine.

British Airways has temporarily suspended flights to and from Liberia and Sierra Leone until 31 August 2014 because of the health crisis, the airline said in a statement. It follows a similar suspension by two regional air carriers last week.

The virus spreads by contact with infected blood and bodily fluids. The current outbreak is killing between 50% and 60% of people infected.

There is no cure or vaccine for Ebola - but patients have a better chance of survival if they receive early treatment.

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Airports in Nigeria are now screening passengers for Ebola on arrival

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Health workers have to wear protective suits to ensure they do not catch the virus

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Kent Brantly was able to walk from the ambulance when he arrived at the Atlanta hospital on Saturday

Dr Anthony Fauci, director of the US National Institute of Allergy and Infectious Diseases, says clinical trials are to start in September on an Ebola vaccine that has shown promising results during tests on animals.

"By the middle to end of 2015, we'll be able to have some vaccine - at least to vaccinate health workers - who put themselves at considerable risk when they take care of these patients," he told the BBC's Newsday programme.

The US aid workers were treated with the ZMapp serum before their evacuation from Liberia.

Ms Writebol is in a special isolation ward at Atlanta's Emory University Hospital, where Dr Brantly is being treated by infectious disease specialists.

Three companies, the US government and the Public Health Agency of Canada are behind the experimental drug.

"ZMapp was first identified as a drug candidate in January 2014 and has not yet been evaluated for safety in humans. As such, very little of the drug is currently available," Mapp Biopharmaceutical said in a statement.

"Mapp and its partners are co-operating with appropriate government agencies to increase production as quickly as possible."

Ebola has initial flu-like symptoms that can lead to external haemorrhaging from areas like eyes and gums, and internal bleeding which can lead to organ failure.

In another development, the World Health Organization is to hold an emergency meeting in Geneva starting on Wednesday to discuss new measures to tackle the Ebola outbreak.

The meeting will decide whether to declare a global health emergency which could involve imposing travel restrictions on affected countries.

Ebola virus disease (EVD)

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Symptoms include high fever, bleeding and central nervous system damage

Fatality rate can reach 90% - but the current outbreak is about 55%

Incubation period is two to 21 days

There is no vaccine or cure

Supportive care such as rehydrating patients who have diarrhoea and vomiting can help recovery